1. FIELD OF THE INVENTION
The present invention relates to a method for forming a fiber-reinforced metal sheet. More particularly, it relates to a method for forming a fiber-reinforced metal sheet in which press rollers and a plurality of laser beams are used.
2. DISCUSSION OF THE BACKGROUND
As a method for forming a fiber-reinforced metal sheet, there is a proposal to manufacture a thin sheet of the fiber-reinforced metal by using a plurality of wire preforms as an elemental material, each being formed by combining fibers and a matrix and the wire preforms being arranged in one direction in a side-by-side relation; and by pressing them immediately after a laser beam has been irradiated onto the regularly arranged wire to thereby bond the wire preforms together (The Summary of Lecture in the Japan Metal Academy, p. 592, October, 1982).
FIG. 3 shows schematically the idea of forming a fiber-reinforced metal sheet.
A plurality of wire preforms 1 are regularly arranged on a supporting plate in a parallel relation in the longitudinal direction of the supporting plate 5, and are transferred to a pair of press rollers 2. Just before pressing the wire preforms 1 by the press rollers 2, a laser beam 3 irradiates to the wire preforms 1. The wire preforms 1 are heated to a temperature to cause the matrix in the preforms to melt by the irradiation of the laser beam 3. The wire preforms 1 are pressed by the rollers in an elevated temperature condition, whereby a strong pressing force is applied to the wire preforms to thereby form a one-piece structure. In order to simultaneously join the plurality of wire preforms 1, a light-focusing device 4 is provided so that the laser beam 3 can be uniformly emitted to the plurality of wire preforms. In this case, it is necessary that the laser beam 3 having a distribution of a linear form (FIG. 4a) or a trapezoidal form (FIG. 4b) is given to the wire preforms by the beam focusing device 4.
In the conventional method of forming the fiber-reinforced metal sheet, a CO.sub.2 laser or a YAG laser has been solely used to produce a laser beam.
For the wire preform, silicon-carbide-fiber-reinforced aluminum and carbon-fiber-reinforced aluminum are used. These materials constitute an elemental material in the method of forming the fiber-reinforced metal sheet according to the present invention. However, when a laser beam is used as a heating source for wire preforms having an aluminum matrix, the following disadvantage is found.
When a CO.sub.2 laser is used, the reflection factor of metals constituting the matrix of the wire preforms is high as shown in Table 1. For instance, the reflection factor of aluminum is as high as about 97%. Namely, the absorption factor of the CO.sub.2 laser becomes remarkably low. Accordingly, a remarkably large power of irradiation is needed in comparison with energy required to heat the wire preforms to a temperature necessary to form the wire preforms in one piece by the rollers. On the other hand, the absorption factor of the CO.sub.2 laser in the wire preforms is strongly influenced by factors such as the surface properties of the wire preforms and the distance between the adjacent wire preforms. Accordingly, when the absorption factor is extremely low, the condition of joining the wire preforms is greatly affected by the physical properties, whereby the condition of forming the fiber-reinforced metal sheet becomes unstable.
TABLE 1 ______________________________________ Reflection factor of major metal/laser YAG laser, wavelength CO.sub.2 laser, wavelength Metal of 0.9-1.1 .mu.m of 0.9-1.1 .mu.m ______________________________________ Al 73.3 96.9 Cu 90.1 98.4 Fe 65.0 93.8 Mg 74.0 93 C 26.8 59.0 ______________________________________ (Laser-applied technique handbook, Asakura (1984) p.80)
On the other hand, when the YAG laser is used independently, the reflection factor of the YAG laser by aluminum as the matrix in the wire preforms is lower than that of the CO.sub.2 laser although the reflection factor of the YAG laser is still high. However, an oscillating device having a large output can not be obtained by the YAG laser in comparison with the CO.sub.2 laser. For instance, the maximum output obtained by the CO.sub.2 laser oscillating device is 20 kW, while the maximum output by the YAG laser is 0.6 kW. Accordingly, when a laser beam is spread out in order to simultaneously join a number of wire preforms, the power of irradiation per unit surface area is insufficient.